1. Field of the Invention
The invention relates to field of disk drives and more particularly to the writing of servo tracks onto the disks during manufacture.
2. Description of the Related Art
Disk drive servo tracks are typically written onto a blank magnetic disk after the disk drive has been substantially assembled. While there are many known methods for writing servo tracks onto a blank magnetic disk, the most common methods include the use of a laser interferometer to control a picker that attaches to the ARM assembly of the disk drive and steps the arms and the heads, attached to the end of the arms, across the disk while writing the servo patterns.
In a mechanically perfect world, servo tracks would be written in perfectly concentric circles as the servo track writer steps the ARM assembly across the disk. However, servo track writers are not mechanically perfect. The resultant imperfections, from whatever source, result in the servo tracks “wandering” from an idealized track center. Tracks spacing is limited by degree to which the written circle track “wanders” from the idealized track center in a nonuniform way.
A significant mechanical source for imperfections in the servo write process is the “run out” of the spindle motor. “Run out” is the amount of radial excursion of the motor in response to dynamic forces on the motor. In a fluid dynamic bearing (“FDB”) motor, the degree of run out is primarily related to the lack of radial stiffness of the fluid dynamic bearing. The stiffness of a fluid bearing is generally related to 1) the size of the gap between bearing surfaces, and 2) the viscosity of the bearing fluid and 3) to the rotational velocity of the motor: The larger the gap, the less viscous the fluid or the slower the rotational velocity (to a point), the looser the bearing.
FIG. 1 shows the general relationship between bearing stiffness and disk rotational velocity of a disc drive FDB motor. It charts the inverse of stiffness (e.g., microinches per 1-g excitation), 1/k, vs. the frequency, f, of rotation for two different bearings. The top curve 10 shows the profile of a relatively loose bearing. The bottom curve 20 shows the profile of a stiffer bearing. Both have peaks, 12 and 22 respectively, and at approximately half the frequency of rotation of the motor.
Increasing radial stiffness of the FDB bearing therefore reduces FDB run out. However, there is a trade-off between bearing stiffness and power consumption: the greater the stiffness, the higher the power consumption. Higher power consumption is extremely undesirable in disk drives for a variety of reasons.
Therefore there is a need to increase FDB bearing radial stiffness without increasing power consumption in order to permit the writing of servo tracks at higher track densities.